1. Field of the Invention
The present invention relates to a backlight unit of a liquid crystal display (LCD) device, and more particularly, to a direct type backlight unit having U-shaped lamps and which obtains light uniformity by preventing a dark line from being generated in the center of an LCD panel of the LCD device.
2. Discussion of the Related Art
Recently, various display devices have been actively researched and studied, for example, a liquid crystal display (LCD) device, a field emission display (FED) device, an electro-luminescence display (ELD) device, and a plasma display panel (PDP). Among these display devices, the LCD device has been most widely used because of the advantageous characteristics of thin profile, lightness in weight, and low power consumption. The LCD device is continuously in demand for use in notebook computers, desktop monitors and liquid crystal televisions.
Because the LCD device does not itself generate light, the LCD device requires an additional light source. In the case of a transmitting type LCD device, it is necessary to provide an additional light source such as a backlight unit, which emits the light toward the rear of an LCD panel. The backlight unit is classified into a direct type backlight and an edge type backlight, according to the arrangement of light-emitting lamps.
In the edge type backlight unit, a linear light-emitting lamp (cold cathode fluorescent lamp or hot cathode fluorescent lamp) having a tubular shape is provided at one side of an LCD panel. A transparent light-guiding plate is also provided to guide the light emitted from the light-emitting lamp to the entire surface of the LCD panel.
In the case of the direct type backlight unit, a light-emitting lamp is selectively provided below an LCD panel, whereby the light emitted from the light-emitting lamp is uniformly diffused to the entire LCD panel by light-diffusion sheets provided between the light-emitting lamp and the LCD panel. The direct type backlight unit is advantageous in that it permit the device to have a lightweight thin profile with high luminance and uniform light distribution.
A direct type backlight unit according to the related art will now be described with reference to the accompanying drawings. First, an LCD device having a direct type backlight unit according to the related art will be described. The LCD device includes an LCD panel, polarizing sheets, a direct type backlight unit, a case, and a bezel unit. The LCD panel is comprised of lower and upper substrates being bonded to each other, and a liquid crystal layer formed between the lower and upper substrates. The polarizing sheets are adhered to the lower and upper surfaces of the LCD panel, to transmit only light of a predetermined orientation. The direct type backlight unit is provided at a predetermined interval from the LCD panel to provide the light to the LCD panel. The case covers the external surface of the backlight unit, to support and fix the LCD panel and the direct type backlight unit. The bezel unit, made of stainless steel, is adhered to the external of the case. The bezel unit covers the circumference of the LCD panel except for an effective area for displaying the image.
As shown in FIG. 1, the direct type backlight unit includes a plurality of straight light-emitting lamps 111, an outer case 103, and optical sheets including a light-diffusion sheet 115, a prism sheet 116 and a protection sheet 117. The light-emitting lamps 111 are formed of cold cathode fluorescent lamps CCFL, with both ends thereof inserted into holes formed at both sides of the outer case 103. The optical sheets including the light-diffusion sheet 115, the prism sheet 116 and the protection sheet 117 are positioned between the light-emitting lamps 111 and an LCD panel (not shown).
The optical sheets minimize the silhouette of the light-emitting lamps 111 from being shown on a display surface of the LCD panel (not shown), and provide a light source with uniform luminance. In addition, a reflective sheet 114 is provided below the light-emitting lamps 111 for concentrating the light emitted from the light-emitting lamps 111 to the display part of the LCD panel, thereby improving the light efficiency.
However, the backlight unit of the linear shape reaches the limit on the division drive of the LCD device. To perform the division drive of the LCD device, in a state of dividing the display area into a plurality of parts, the quantity of light of the backlight unit increases in the part of the bright image, and the quantity of light of the backlight unit decreases in the part of the dark image. In this case, a light-emitting lamp 211 is formed in a U-shape, as shown in FIG. 2.
FIG. 2 is a schematic view showing a display area divided into eight parts. To form the U-shaped light-emitting lamp 211, as shown in FIG. 2 and FIG. 3, a fluorescent substance 205 is coated on an inner surface of a U-shaped glass tube, and then a mixed inert gas or a discharge gas such as hydrargyrum Hg is injected into the U-shaped glass tube. After that, both ends of the U-shaped glass tube are sealed. In this state, electrodes 208 are formed at both ends of the glass tube. Also, power supplying wires 209 are respectively connected with both electrodes 208 of the glass tube, to supply the power to the light-emitting lamp 211. The power supplying wires 209 are connected with an inverter 206 by a connector 207.
At this time, the U-shaped light-emitting lamps 211 are symmetrically arranged such that the curved part of each of the light-emitting lamps is positioned in correspondence with the center of the display area. In this case, the quantity of light in the center of the display area varies according to the distance between the curved parts of the opposite U-shaped light-emitting lamps.
If there is a large distance between the curved parts of the opposite U-shaped emitting lamps, a dark line (A) is generated in the center of the display area. Conversely, if there is a small distance between the curved parts of the opposite U-shaped light-emitting lamps, a bright line is generated in the center of the display area.
On the division drive of the U-shaped light-emitting lamps in the symmetric arrangement of the light-emitting lamps, a dark line or a bright line may be generated in the center of the display area, thereby deteriorating the picture quality.